This invention relates to a hydraulic circuit used in construction machines, industrial vehicles and likes.
A typical example of such hydraulic circuit is disclosed in U.S. Pat. No. 4,693,272 and it will be described first with reference to FIGS. 1 and 2. As shown in the drawings, the circuit is arranged such that a plurality of direction changeover valves 54 connected with an actuator 53 are connected to the outlet of a pump 52 of variable discharge type whose discharge quantity varies with a pilot pressure acting to a regulator 51 and each direction changeover valve 54 is provided with a pressure compensation valve 55 and a shuttle valve 56 for selecting the maximum load pressure, thereby causing the load pressure selected by the shuttle valve 56 to act to the regulator 51 and a spring chamber 57 of the pressure compensation valve 55.
In operation of the above-mentioned hydraulic circuit, the shuttle valve 56 of each direction changeover valve 54 is connected through passages 59a, 59b and 59c of a spool 58 to a tank passage 60 when the spool 58 is in its neutral position as shown in FIG. 1. Accordingly, a tank pressure acts to the regulator 51 of the variable discharge pump 52 and, therefore, the pump 52 produces at its outlet an oil pressure responsive to a pressing force of a spring disposed in the regulator 51. Accordingly, in the state of FIG. 1, the circuit is held in such a condition that low pressure oil is confined within passages 61 and 62 of the direction changeover valve 54.
When the spool 58 of the direction changeover valve 54 is moved rightwards from the position as shown, the passage 59a is first closed by a slide hole of the spool 58 and a load passage 63 and a first supply passage 64 are connected with each other. With such connection of the load passage 63 and the first supply passage 64, the load pressure of the actuator 53 acting on the load passage 63 acts also on the first supply passage 64. This load pressure further acts through the shuttle valve 56, a high pressure selection passage 65 and a passage 68 to the regulator 51. Therefore, the variable discharge pump 52 delivers an oil pressure which is higher than the load pressure of the actuator 53 by an amount corresponding to the spring force in the regulator 51. If the spool 58 is moved further rightwards, the entrance passage 61 is connected to a second supply passage 67 through a metering iris 66 and, at this time, the oil pressure of the second supply passage 67 in the upstream side of the pressure compensation valve 55 becomes a pressure corresponding to the load pressure of the actuator 53 acting on the pressure compensation valve 55 since the load pressure of the actuator 53 has acted already through the high pressure selection passage 65 and the passage 68 to the spring chamber 57 of the pressure compensation valve 55. Accordingly, the pressure difference between the side of the entrance passage 61 (upstream side) and the side of the second supply passage 67 (downstream side) of the metering iris 66 becomes a pressure difference responsive to the spring force of the regulator 51. Therefore, the oil quantity passing the metering iris 66 becomes to have a value responsive to the aperture of the metering iris 66 (or the amount of movement of the spool 58).
While the above description has been made on the operation of the direction changeover valve as shown when its spool 58 is moved, similar operation will take place when a plurality of direction changeover valves are operated at the same time. More particularly, at this time, the highest one of the actuator loads connected to the respective direction changeover valves is selected and applied from the high pressure selection passage 65 through the passages 68 and 69 to the spring chamber 57 of the pressure compensation valve 55 of each direction changeover valve 54. Accordingly, the oil pressure difference between the upstream and downstream of the metering iris 66 of each direction changeover valve becomes a differential pressure responsive to the spring force of the regulator 51 and, therefore, the oil quantity passing each direction changeover valve has a value responsive to the amount of operation of each direction changeover valve.
When such hydraulic circuit is applied to an actuator of a construction or industrial machine, the variable discharge pump 52 which is used as its oil pressure source is generally driven by an engine and located near the engine. However, the direction changeover valve for feeding pressurized oil discharged from the pump to a plurality of actuators are often located far from the pump. Therefore, the outlet of the pump and the inlet of each direction changeover valve are connected through a hydraulic piping which produces a substantial pressure loss. Thus, there has been such a problem in that the pressure loss may affect the flow control function of each direction changeover valve and this problem will be described in more detail below.
When any of the direction changeover valves is operated in the above-mentioned hydraulic circuit, the load pressure of the operated actuator 53 acts on the regulator 51 of the variable discharge pump 52 to produce an oil pressure corresponding to the load pressure. As the pressurized oil is supplied through a piping to the direction changeover valve, the oil pressure of the supply passage of the direction changeover valve is lower than the oil pressure at the outlet of the variable discharge pump 52 by a value corresponding to the piping resistance. Accordingly, the pressure difference formed by the spool 58 of the direction changeover valve across the metering iris 66 becomes lower than a pressure corresponding to the spring force of the regulator 51 by the above-mentioned value corresponding to the piping resistance. The piping resistance increases with increase of the rate of flow therethrough and, therefore, if the aperture of the metering iris 66 increases, the pressure difference across the metering aris 66 will decrease correspondingly. Accordingly, it becomes impossible to control the flow rate in response to the movement of the spool 58. This problem may result in the following trouble, for example. When a plurality of direction changeover valves are operated at the same time and the amount of operation is constantly maintained, if the amount of operation of one of the direction changeover valves is increased (or decreased), the oil quantity flowing from the variable discharge pump to the direction changeover valve will increase (or decrease) correspondingly. Therefore, as described above, the piping resistance will increase (or decrease) to decrease (or increase) the pressure difference across the metering iris 66 of each direction changeover valve, thereby decreasing (or increasing) the flow rate. Accordingly, when two direction changeover valves are operated at the same time, the operation of one direction changeover valve may result in instantaneous reduction (or increase) of the speed of the actuator 53 connected to the other direction changeover valve.
Accordingly, an object of this invention is to solve the above-mentioned problem in the prior art and provide an improved hydraulic circuit which enables stable flow rate control.